Require `chandler` to be never null by installing `[]()noexcept{}` as the handler if given a null pointer.
This lets us remove the double test in `assertion`.
Allows independently controlling handling of different categories of bugs, such as bounds checks vs. null checks.
User-extensible: Companies can instantiate their own `contract_group` objects for their own categories of contract checks, including for distinguishing contract "levels" like `Normal` vs. `Audit` by just creating those two groups that can then be controlled independently or in combination.
This is not specified in the C++ Core Guidelines as part of GSL, so it should not be documented or supported with a GSL name. And since we don't use it in the GSL repo either, we should just remove it directly. For more about why assumptions are a dangerous blunt instrument, including that they actively eliminate contracts checks including GSL's `Expects` and `Ensures`, see [P2064](http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p2064r0.pdf).
In microbenchmarks I tried, using `__builtin_expect` to indicate which branch was likely/unlikely was never faster, and often slower, than just omitting. In GSL, this is used only for `Expects` and `Ensures` which are nearly always true, and all processors have long been great at predicting such branches well.
